1. Field of the Invention
The present invention relates to a gas-barrier polyurethane resin, and an adhesive for laminate, a gas-barrier film and a paint that contain such a resin. More specifically, the invention relates to (A) a two-part liquid curable polyurethane resin composition capable of exhibiting not only a high gas-barrier property but also a high adhesion property, especially an excellent adhesion property to various polymers, papers, metals, etc., after boiling and retort treatments, as well as an adhesive for gas-barrier laminate containing such a composition; (B) a heat-curing gas-barrier polyurethane resin that is useful as a packaging material for foodstuffs or drugs for the purpose of preserving contents thereof by preventing various gases such as oxygen, water vapor and gaseous aroma components from penetrating therethrough, and not only maintains a high gas-barrier property especially even after boiling and retort treatments under a high humidity condition, but also exhibits an excellent adhesion property to a base film, as well as a gas-barrier film containing such a resin; and (C) a paint that can be extensively used in various applications such as coating materials for rust-proofing, corrosion-resisting and aesthetic decorating purposes, and packaging materials for foodstuffs or drugs which are required to show a high gas-barrier property.
2. Description of the Prior Art
In recent years, packaging materials have been predominantly prepared from composite flexible films using different kinds of polymer materials in combination because of their good strength, goods-keeping ability, working suitability, printability for advertising effects, etc. The composite flexible films generally include an outer layer made of a thermoplastic resin film, etc., serving for protecting goods, and a sealant layer made of a thermoplastic resin film, etc. These layers are laminated by a dry-lamination method in which the sealant layer is bonded to the laminated film layer via an adhesive applied to the film layer, or by an extrusion lamination method in which the sealant layer made of molten plastic film is press-bonded to the laminated film layer on which an anchor coat agent may be applied, if required. As the adhesive used in these methods, there have been predominantly adopted two-part liquid polyurethane-based adhesives usually composed of a main ingredient containing an active hydrogen-containing group such as hydroxyl group, and an isocyanate group-containing curing agent, in view of a high adhesion property thereof.
However, the conventional polyurethane-based adhesives in themselves exhibit no gas-barrier property. Therefore, when these adhesives are applied to packaging materials requiring a gas-barrier property, it is necessary to separately laminate various gas-barrier layers such as a polyvinylidene chloride (PVDC) coating layer, a polyvinyl alcohol (PVA) coating layer, an ethylene-vinyl alcohol copolymer (EVOH) film layer, a meta-xylyleneadipamide film layer and an inorganic deposited film layer on which alumina (Al2O3), silica (Si) or the like is vapor-deposited. Of these layers, the PVDC-coated film is known as a laminated film having a high barrier property to oxygen and water vapor and, therefore, have been extensively used as various packaging materials for foodstuffs. However, recently, there occurs such a problem that the PVDC-containing packaging materials generate dioxins upon disposal thereof. Therefore, it has been strongly required to replace the PVDC materials with other materials.
Further, there is a recent tendency that plastic films or containers are predominantly used as packaging materials for preserving contents, because of good transparency, light weight, inexpensiveness, etc. The plastic films used for packaging foodstuffs, drugs, cosmetics, etc., are required to have a gas-barrier property to various gases, transparency, resistance to retort treatment, impact resistance, flexibility, heat sealability and the like. In particular, in order to keep properties and quality of contents to be packaged, the films are required to show a high gas-barrier property to oxygen and water vapor even under a high humidity condition or after retort treatment.
Such gas-barrier packaging materials are usually constituted by a laminate including a flexible polymer film base layer, a gas-barrier layer, a flexible polymer film sealant layer, etc. It is known that among these layers, the gas-barrier layer is made of a gas-barrier material such as polyvinylidene chloride (PVDC) coat or film, an ethylene-vinyl alcohol copolymer (EVOH resin) film, a meta-xylyleneadipamide film, an inorganic deposited film deposited with alumina (Al2O3), silica (Si) or the like, a polyvinyl alcohol (PVA) coat, etc. These gas-barrier materials are selectively used according to properties thereof, kinds of contents to be packaged, or applications thereof. Among them, it is known that the PVDC-coated films have a high gas-barrier property to oxygen and water vapor. Since PVDC shows substantially no water vapor absorption and a good gas-barrier property even under a high humidity condition, various base films are coated with the PVDC irrespective of their water vapor permeability, and the PVDC-coated films are employed for packaging various foodstuffs in the form of either dried or hydrated products. These packaging materials for domestic use are disposed of after use as ordinary wastes. In order to provide packaging materials that are favorable in view of both disposal costs and environmental protection, it has been strongly required to develop new resins that are excellent in both gas barrier property and adhesion property.
Under this circumstance, as alternative techniques, it is known to use a film made of a saponification product of ethylene-vinyl acetate copolymer (EVOH resin), a polyvinyl alcohol (PVA)-coated film, an inorganic deposited film obtained by depositing silica or alumina on a flexible polymer film, etc. However, the EVOH resin film and the PVA-coated film suffer from remarkable deterioration in oxygen-barrier property when exposed to water under a high-humidity condition or when being subjected to boiling or retort treatments. Also, the inorganic deposited film whose gas barrier layer is formed by depositing hard inorganic compounds thereon, tends to undergo formation of pinholes in the gas barrier layer upon flexing, resulting in remarkable deterioration in its gas-barrier property. Further, the production of such a vacuum-deposited film requires a large scale apparatus as compared to the production of ordinary coating films or laminated films, thereby leading to high production costs.
Also, the polyurethane resins exhibit many excellent properties such as good adhesion to various base materials, heat resistance, chemical resistance, electric properties, mechanical properties and anti-weathering property by themselves, as compared to other resins, and are, therefore, extensively used in various applications such as paints for rust-proofing, corrosion-resisting and aesthetic decorating purposes, adhesives for civil engineering or building construction, etc. Since the polyurethane resins usually used for paints have substantially no gas-barrier property, the polyurethane resin paint must be coated with a large thickness, laminated with other materials, or used together with fillers in order to prevent corrosive factors from penetrating therethrough.
On the other hand, as gas-barrier polyurethane resins or related techniques, Japanese Patent Application Laid-open No. 2001-98047 discloses a gas-barrier polyurethane resin and a gas-barrier film containing the resin, Japanese Patent Application Laid-open No. 7-112518 (1995) discloses a gas-barrier laminated stretched film using a polyurethane-based thermoplastic elastomer, and Japanese Patent Application Laid-open No. 10-168305 (1998) discloses a polyurethane composite material that is enhanced in gas-barrier property by adding layered clay mineral thereto.
However, these films have no adhesion properties therebetween and, therefore, are used only in the similar applications to those of the conventional gas-barrier films. Accordingly, in the packaging materials requiring a gas-barrier property, when the gas barrier film is laminated on the ordinary film, both surfaces of the gas-barrier film must be coated with an adhesive, resulting in not only disadvantages such as high production costs of the laminated film and complicated lamination process thereof, but also adverse influences on environments due to increased amount of wastes which have been noticed as recent social problems. Therefore, in order to provide packaging materials that are favorable in view of both production costs and environmental protection, it has been strongly required to develop resins that are excellent in both gas-barrier property and adhesion property.
In addition, the polyurethane resins disclosed in Japanese Patent Application Laid-open Nos. 7-112518 (1995) and 10-168305 (1998) are still insufficient in gas-barrier property when applied to packaging films, and are deteriorated in gas-barrier property especially when used under a high humidity condition. Therefore, it is desirable to further improve the gas-barrier property of these resins. Whereas, Japanese Patent Application Laid-open No. 2001-98047 discloses the gas-barrier polyurethane resin capable of showing an improved gas-barrier property even under a high humidity condition, as well as a gas-barrier film containing such a resin. However, the polyurethane resin is a thermoplastic resin and, therefore, fails to exhibit a sufficient gas-barrier property when exposed to a still higher humidity condition or after boiling and retort treatments under a high humidity condition. Thus, at preset, no alternative materials that are usable instead of PVDC have been obtained in view of properties and production costs thereof.